When we say we list all of your acting credits, we don't mean all of the acting credits that are definitely confirmed by a third party. Yes most actor websites will let you add yourself to an existing project, but for many up-and-coming actors that may not be enough. If you're new to the acting world, or even "somewhat" new, filling up your resume might be tough. You certainly don't want to list your non-acting jobs on there, because even if you work as an administrator for the biggest film studio in LA, it's not an acting skill. It might be a great stepping stone or a way to get your foot in the door, but it shouldn't go on your resume.
If, however, you had a small role in an even smaller play, or have been a featured extra on that one TV show, it may be worth letting people know about it. It may not say that you're the greatest actor on the planet, but it definitely says that you are serious about acting. And trust me, a lot of the credits that you might get through independent projects will not get approved on your IMDb, but it's worth listing them nonetheless.